Two things all SPFL backup keepers have to contend with that work against them.
1) Obviously, as a backup you aren't playing with your team mates every week, so the understanding isn't there to the same extent as first choice keeper.
2) With the current lack of reserve teams, you get zero competitive games and cannot possibly be match fit without a run in the first team.
I don't know what Neilsen's longest run of consecutive games is, but it can't more than a small handful.
Not to say that we haven't a signed a player that just isn't going to work out (it happens), but when you are an outfield player like a Porter, Higdon or a McManus, you get a couple of months to settle in without the same pressure when you make mistakes.
But while he is our starter, he needs a bit of support.